MySQL Conditional Insert

Question

I am having a difficult time forming a conditional INSERT

I have x_table with columns (instance, user, item) where instance ID is unique. I want to insert a new row

Answer 1

With a UNIQUE(user, item), do:

Insert into x_table (instance, user, item) values (919191, 123, 456) 
  ON DUPLICATE KEY UPDATE user=123

the user=123 bit is a "no-op" to match the syntax of the ON DUPLICATE clause without actually doing anything when there are duplicates.